Article 31229 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal Subject: Comments on "Solar Water Heating" by Bob Ramlow Date: 1 Sep 2008 05:56:54 -0400 Organization: Villanova University Lines: 347 Message-ID: NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1220263016 32760 153.104.44.130 (1 Sep 2008 09:56:56 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Mon, 1 Sep 2008 09:56:56 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:31229 I've been finding curiousities in Ramlow's 2006 book "Solar Water Heating, a comprehensive guide to solar water and space heating systems"... Dan writes: >> page curiousity >> >> 7 "insulate all your hot pipes..." This will save very little >> energy, since hot water is used in bursts and the insulated pipes >> contain little water, with a short time constant compared to the >> burst intervals. > Insulating hot water pipes saves energy! Not much, with a long payback. Airlock vestibules don't save much either, unless you live in a department store :-) >> 9 "in almost every case, a solar water heating system is the best >> place to start. It provides a higher return on your investment than >> any other renewable energy system..." I disagree. A low-mass >> polycarbonate lean-to sunspace or thermosyphoning air heater is a >> better place to start. > I have one and like it but would place solar hot water before hot air. At $40 vs $1/ft^2? You can do both, with a sunspace, but you can't live in a water heater. And why do solar water heaters have to cost $7K vs $1K? >At a recent American Solar Energy Association meeting the mantra was >that every building should have a solar hot water system. I doubt the 50 people in our 2004 ASES solar heating workshop in Portland OR would echo that mantra. >> 25 Only low-iron tempered glass has stood the test of time? :-) > > Yes, that's true. Rubbish. Plain old glass lasts a long time, and $1/ft^2 polycarbonate lasts 20 years. A double-glazed polycarb panel can be reversed to expose the indoor face to the outdoors for another 20 years. I'm 62 now... >> 29 Fig 3.7 belies the words "they are still not more efficient >> than flat plate collectors in most temperatures..." during cloudy >> conditions... > You have to look at the entire system performing year-round, > not just on cloudy days. But Bob was only talking about cloudy days. >> 31 No mention that concentrating collectors don't need to track, >> nor that they only work well in sunny climates? > > All but a very few solar hot water systems do not track! Some concentrators don't move at all. The most serious mistake was making the outer container of the receiver of plywood. We thought that the plywood would be sufficiently insulated from the copper panel which was the receiver proper, that it would not get too hot. The copper panel was separated from the plywood by 4" of fiberglass insulation. Nevertheless, the plywood caught fire and the unit was completely destroyed. We suppose this is a success, of sorts. The copper panel which was plated with chrome black to provide a selective surface originally had a copper tube fastened to the back by a high melting point soft solder. When we first attempted to operate the unit, the soft solder melted, and the tubing became detatched from the panel. We attempted to repair this failure by silver soldering copper bars to the copper tube and screwing the bars to the plated copper sheet. This worked, after a fashion... The glass window was originally tempered glass. This shattered due to thermal shock. We replaced it with ordinary window glass. This cracked due to thermal shock, but we were able to hold it in place well enough to make some measurements... The measurements we were able to make before the fire generally confirmed our thinking concerning the design... from "A solar collector with no convection losses," (a downward-facing receiver over a 4:1 concentrating parabolic mirror), by H. Hinterberger and J. O'Meara of Fermilab, in "Sharing the Sun," A joint ASES/SESC conference, August 15th-20th, 1976(?), Winnepeg, Volume 2, pp 138-145. >> 32 Plastics are not affected by chlorine? :-) > > They are but minimally, which is why swimming pool systems use them. Chlorine is death to many plastics. >> 33 No mention of glazed transpired mesh air heaters? > > It's not a book about air heaters. Page 33 is about air heaters. >> 36 No mention of STSS tanks or site-built equivalents... >> 37 "Don't use plastic or rubbber lined tanks for any solar >> thermal application.,," Ohoh. Better warn Sven Tjernagel, who's been >> selling STSS EPDM-lined solar heat storage tanks for at least 30 years. > > This is a debate worth staying out of! They work fine up to 170 F. >> 49 No mention of floating ball check valves. > > He mentions all relevant valves. No. Floating ball check valves are important parts in many solar water heating systems. >> 60 Why are all open-loop solar water heaters direct systems? >> What about a pressurized pipe coil in an unpressurized draindown >> heat storage tank? That is an open loop indirect system. > The panels are on the roof, storage below. Huh? >> 61 Why do we need 2-tank systems? > > Storage and heat exchange. They can both happen in one tank. >>And where is the "slow-draindown" system with a pump that works >>harder to start? > > The "hard" part is a split second high amperage burst. A pump might only move 1 gpm up through a collector, until the return pipe fills, making a partial vacuum, with more flow and no current burst. Grainger's $189 4PC84 178 W pump can only move 3 gpm up 20', but it can move 21.5 gpm up 10', after it fills a return pipe, like a $201 4PC82 230 W pump. >> 72 Moving water is hard to freeze? :-) > > Yes, old school stuff Nick. Turn the faucet on at a slow trickle when > temperatures drop and you prevent freezing. Water freezes at 32 F, period. A trickle of fresh ground water can keep a pipe above 32 F. >> 79 Shunt loads vs stagnation again... ("Here is your new car. >> Whatever you do, don't ever turn off the engine.") > > That's not what Bob is saying. I think he is, over and over. >> 82 What's wrong with a bubble in the pressure gauge? >> Why is it upside-up in Fig 5.11? :-) > > Easier to read than upside down. A bubble won't change the pressure reading. More, beginning with solar space heating systems: page curiosity 96 "It is impractical to have a heat storage mass that can hold enough heat to do the job..." But PE Norman Saunders has been designing practical inexpensive solar houses like this since 1946. He estimates that some clients will only need to "purchase heat" every 35 years. "On sunny days that are not as cold we will have excess heat and could overheat something..." But winter overheating is easy to fix, just turn on a fan or open a window. Or use smarter controls that don't overheat the house in the first place. 98 Drainback systems are not unsuitable in freezing climates. 100 Why does gravity eliminate the possibility of remote-mounting a collector array? :-) 101 Solar heat is not "compatible with all existing heat delivery systems," and diversion loads are avoidable. 103 STSS EPDM-lined tanks (which easily fit through doorways when compressed in their crates) have lasted a lot longer than 15 years, and steel tanks can last a long time with a non-toxic corrosion inhibitor, eg a 0.5% solution of ACI-100 (mostly sodium silicate) from D.W. Davies. 104 Caps on heat exchanger frames to avoid wearing holes in EPDM liners are good, but a $60 13-gallon 1"x300' HDPE plastic pipe coil heat exchanger seems better, with a 140 F max tank temp... 2" of foamboard insulation isn't much. What about homemade heat storage tanks? 105 Why 1' of heat exhange tubing (what size?) per 4 ft^2 of collector? Why the weird bulkhead fitting arrangement. 106 Why do we need a high-head full-flow pump for drainback, vs a 2-speed pump or 2 pumps in series with 1 turning off after the return pipe fills in a slow drainback system? Why 4' of friction head? A separate drainback tank with a heat exchanger below seems like a poor way to avoid a high-head pump. 107 Why not mingle solar and conventional HVAC fluids? No mention that efficient solar heaters have lower fluid temps which won't work well with high-temp radiators. Why compromise comfort with 2 thermostat setpoints instead of using smarter controls with a single setpoint? 108 The min tank temp depends on the indoor and outdoor temps and wind and indoor electrical usage and the amount of sun shining into windows, vs the fixed 85 F mentioned. 109 Why blowers and ductwork, vs passive convectors and floor returns near outside walls as in old-fashioned gravity furnaces and Harry Thomason's houses? Why not turn on a blower with a line-voltage thermostat? 110 A summer/winter switch seems awkward. PE Norman Saunders says a well- designed solar house is just as likely to need cooling as heating at any instant in wintertime. He provides a cool source (20K pounds of rocks in the basement) as well as a heat source (10K pounds of water containers in the attic) in his 100% solar-heated houses. 111 Standalone seems good. Bigger wire? :-) Fin-tube baseboard and cast- iron radiators won't work well with efficient low-temp solar water. 113 Rich Komp's Maine house has a high-mass solar-warmed air hypocaust, a 4" concrete slab over 8x8x16" hollow blocks over a vapor barrier. A small PV-powered fan pushes warm air down from the top of the room under the slab when the sun shines. Ernie the ermine takes care of vermin. High-mass solar systems are not necessarily heated with closed-loop antifreeze. 114 The book describes 2' of sand as if it were a seasonal heat store, but with no insulation above, at say, 80 F when fully-charged (if we have to live inside the heat store), a 1200 ft^2 house with 504K pounds of sand with a specific heat of 0.191 Btu/F-lb (vs 1 for water) with C = 0.191x504K = 96.3K Btu/F and a low thermal conductance U = 200 Btu/h-F would have a time constant RC = C/G = 481 hours. At the average 16 F January temp in Madison, WI, it would cool from 80 to 70 F in -481ln((70-16)/(80-16)) = 82 hours, about 3 days. Or less, if we include the sand resistance. One 105 cubic foot of sand can store 0.191x105 = 20 Btu/F, the same as 20/8.33 = 2.4, not 4.8 gallons of water, and water is about half, not twice as dense as sand, which is why we don't see sand floating on water :-) 115 "The first thing to realize is that it may take an entire year to get the sand bed dry and hot." No... it might take a week, and it could store 100x0.4x62.33/20 = 75% more heat if wet, with 40% water by volume. OTOH, we might store heat in a 26 million gallon "water tank"... The Lyckebo [Sweden] system is a cavern of 100,000 m^3 capacity, cut out of bedrock using standard mining methods, of cylindrical shape, with a central column of rock left to support the overhead rock. The cavern is about 30 m high and its top is about 30 m below ground level. It is water filled, and inlet and outlet pipes can be moved up and down to inject and remove water from controlled levels. The water is highly stratified with top to bottom temperatures of about 80 to 30 C. Figure 8.7.2 shows temperature profiles in the store at various dates in the second year of operation... No thermal insulation is used, and there is a degree of coupling with surrounding rock which adds some effective capacity to the system. Losses occur to a semi- infinite solid and can be estimated by standard methods. Observed losses from this system are higher than those calculated; this is attributed to small but significant thermal circulation of water through the tunnel used in cavern construction and back through fissures in the rock. It takes several years of cycling through the annual weather variations for a storage system of this size to reach a "steady periodic" operation. In the second year of its operation, while it was still in a "warm-up" stage, 74% of the energy added to the store was recovered. from p 404 of section 8.7, "seasonal storage," of _Solar Engineering of Thermal Processes_, by John A Duffie and William A Beckman, 2nd edition, 1991, Wiley-Interscience ISBN 0-471-51056-4 116 Heating floor mass with house windows open doesn't sound efficient :-) 117 "This fresh air is a real bonus..." :-) More manual shunt valves, ick. Why do all high-mass systems need antifreeze and diversion loads? :-) Whatever happened to draindown systems? 118 Hot tubs are better diversion loads than pex buried in soil, but for most people, 103 F is too cool and 105 is too hot. Air can heat water, as in http://www.builditsolar.com/Projects/Sunspace/LowCostHtStorageNathan.pdf Nathan Hurst stored lots of solar heat in water from sunspace air using an inexpensive car radiator with its 20 watt fans, vs "a substantial blower." Air collectors can be more efficient than water collectors, and they can be set up as systems WITH storage, as in the NH CSI HQ building, 100% heated "with 98% sun power and 2% blower power." 119 Why not store heat in an air system? Lots of people have done this successfully, including PE Norman Saunders. Bob fails to mention glazed transpired mesh air heaters again, and says wall-mounted systems can't heat buildings in summertime :-) 120 We don't need no steenkeeng ductwork. 121 Cold air won't flow through a hole unless it is rectangular? :-) Air handling energy can be small, with large flow passages and fans or slow blowers, and some motorized dampers only use 2 watts when moving. Draindown systems don't need freezing heat exchangers :-) 122 "A promising technology that I have hopes for is the use of air rising in a a solar chimney to spin an electric generator." Here's a naive efficiency estimate for these "helio-aero-gravity" towers, huge chimneys with wind turbines at the top surrounded by acres and acres of greenhouses in deserts to make a "solar wind" up the chimney... We can start with an old empirical chimney formula, cfm = 16.6 Av sqrt(HdT), where Av is the vent area at the top and bottom, H is the height difference in feet, and dT is the (F) temperature difference. The airstream's heat power in Btu/h is roughly cfmdT = 16.6H^0.5dT^1.5 for a 1 ft^2 chimney. With a constant power heat source (the acres and acres of greenhouses), this equation naively implies that the taller the chimney, the smaller the temperature difference and the larger the air velocity. Smaller temperature differences are better, representing less waste heat. We might think of chimneys as transformers that increase the air velocity of a heat source and reduce the temperature difference. For a 1 ft^2 chimney, the power density is 16.6H^0.5dT^1.5 Btu/h-ft^2, and the air velocity V = 0.1886H^0.5dT^0.5 mph. Paul Gipe's Wind Power book says wind power density is 0.05472V^3 W/m^2, where V is in mph. He says the best rotors achieve 40% efficiency (vs the 60% Betz limit, which may not apply for a chimney)... 90% efficiencies for the transmission, generator, and power conversion make the wind power density 0.01596V^3 W/m^2 or 0.00506V^3 Btu/h-ft^2. So the heat-to-electrical power conversion efficiency of the chimney is E = 100x0.00506(0.1886H^0.5dT^1.5)^3/(16.6H^0.5dT^0.5) = 0.0002H %, where H is the chimney height in feet. E = 0.002% for a 10' chimney, 0.02% for 100', 0.2% for 1000', and 2% for 10,000'. This doesn't account for the heat loss through the greenhouse glazing or chimney walls, or air friction in the chimney, nor the fact that the weight and cost of a tower increase a lot faster than the height... 124 Why can't a system drain back to a tank in a basement? 125 "Here in Wisconsin... On June 21, the sun... is directly overhead at noon (90 degrees from the horizontal...)" Bob says he lives at 45 degrees north latitude, where the sun would be 90-45+23.5 = 68.5 degrees above the horizon at noon on 6/21. The sun would be directly overhead on 6/21 at 23.5 lat on the Tropic of Cancer, in the part of Wisconsin located in northern Cuba :-) Nick